Porphyrin based macrocycles undergo photochemical processes which are important in natural and artificial photosynthesis and photodynamic therapy. In an effort to elucidate the mechanisms of these processes, we are investigating the electronic structures of the low energy excited states from which these processes occur (the S1 and T1 states) and interactions between the excited states. Our general approach is to alter the electronic environment of the porphyrin via phenyl ring substitution, ligation and solvent variation and note the effect on the photophysical properties (energetics, lifetimes and quantum yields). Time resolved triplet-triplet absorption spectra of para-phenyl substituted zinc(II) tetraphenylporphyrins in several solvents (including ligating solvents) have been investigated. This work has been published (J. Phys. Chem. 99, 1166 (1995)).